A yellow-back is a cheap fiction novel which was published in Britain in the second half of the 19th century. They were occasionally called 'mustard-plaster' novels.
Developed in the 1840s to compete with the 'penny dreadful', Yellow-Backs were marketed as entertaining reading. They had brightly coloured covers, often printed by chromoxylography, that were attractive to a new class of readers, thanks to the spread of education and rail travel.
Routledges were one of the first publishers to begin marketing Yellow-Backs by starting their "Railway Library" in 1849. The series included 1,277 titles, published over 50 years. These mainly consisted of stereotyped reprints of fiction novels originally published as cloth editions. By the late 19th century, Yellow-Backs included sensational fiction, adventure stories, 'educational' manuals, handbooks, and cheap biographies.